JP4319642B2 - Device manufacturing method - Google Patents

Description

  The present invention relates to a device manufacturing method using lithography.

  A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. A lithographic apparatus can be used, for example, in the manufacture of devices such as integrated circuit devices (ICs). In this situation, a patterning means, alternatively referred to as a mask or reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern is applied to a substrate having a layer of radiation-sensitive material (resist). It can be imaged on a target portion (eg consisting of one or more dies) on (eg a silicon wafer). In general, a single substrate will contain a network of adjacent target portions that are successively irradiated. In this specification and in the claims, the expression “substrate” should be construed as representing a substrate carrying or not carrying a processed layer produced in a previous lithographic process step. A known lithographic apparatus scans a pattern with a projection beam in a predetermined direction ("scanning" direction) with a so-called stepper that irradiates each target portion by exposing the entire mask pattern to the target portion in one operation. At the same time, it includes a so-called scanner that irradiates each target portion by scanning the substrate in parallel or antiparallel to this direction.

  To illuminate and pattern the reticle, receive a beam of radiation from a radiation source, called a projection beam, and provide an adjusted beam of radiation having a desired uniformity and intensity distribution on the cross-section Establish a system. The source and the lithographic apparatus may be separate entities, for example when the source is a plasma discharge source. In such a case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transferred from the source to the illumination system with the aid of a radiation collector, for example with a suitable collector mirror and / or spectral purity filter. Is passed. In other cases, for example when the source is a mercury lamp, the source may be an integral part of the device.

  The resolution with which the mask pattern can be copied to the layer of resist is determined by several factors. The main of these factors is the wavelength of the illuminating radiation. Diffraction generated at the mask tends to reduce the resolution of the illumination pattern. Such a reduction in resolution is less with radiation of relatively short wavelengths. Thus, many studies have produced systems with even shorter operating wavelengths. The current goal is to provide a system that operates in the so-called “polar” (E) UV range, ie, wavelengths below 50 nm, eg 13.4 nm or 11 nm. It can be seen that EUV radiation is easily absorbed by the resist material, so that processing utilizing EUV must operate with a very thin resist layer, usually on the order of 100 nm.

  As the wavelength of radiation decreases, the energy of photons impinging on the resist increases and the production of secondary electrons in the resist increases. In fact, at the EUV wavelength, it is secondary electrons that provide the main mechanism for exposing the resist because the photon energy no longer matches the binding energy between the resist molecules. Some background information is useful in understanding this phenomenon and why the secondary electrons can reduce the resolution of the mask transfer.

During exposure of the resist layer, photons impinging on the resist are absorbed by electrons bonded within the atoms of the resist, giving these electrons enough energy to jump out of their individual atomic shells. Vacancy is generated in a specific atomic shell of the atom. This process is known as “photoionization” and the free electrons are known as “photoelectrons”. An important parameter in defining the extent of the exposure area is
-Photon mean free path, determined by absorption in the resist,
-Energy levels in various shells of atoms,
The atomic density of the resist, and the angular distribution of the emitted photoelectrons.

As the electrons move to fill the vacancies left by the photoelectrons, a photon having an energy defined by the difference in valence band between the previous state and the new state is emitted. This process is a form of fluorescence emission. Thus, additional parameters that define the extent of the exposed area are:
-Usable transition between shells (energy difference),
The probability of movement, and the angular distribution of the fluorescence emission.

When a photoelectron collides with a bound electron, the impact can be strong enough to strike the bound electron and provide a “secondary” electron. The electrons move in a new direction with reduced energy. As a result, additional parameters that define the extent of the exposed area are:
− Electron density,
-Probability of scattering / impact movement,
-Angular distribution of scattered primary electrons,
The angular distribution of the secondary electrons produced, and the mean free path of both types of electrons (in principle, the mean free path increases from about 5 nm of 5 eV to about 248 nm, for example, when the electron energy decreases).

  For a more detailed explanation of this theory, see David T. et al. Atwood, “Soft x-rays and extreme ultraviolet radiation (principles and applications)” (Cambridge University Press, 1999 (ISBN 0 521 65214 6)) and p. W. H de Jager, “An instrument for Fabrication and Analysis of Nanostructures Combining Ion and Electron Regulation” (Delft University Press, 1997 (ISBN 90 407 1478) Refer to 9)).

  FIG. 1 schematically illustrates the various processes outlined above. As shown, the energy that actually exposes the resist is generated by any of these processes, particularly as a result of secondary electron generation. For images constrained by point source diffraction, the mean free path of secondary electrons defines a radius that can effectively expose the resist with photochemical effects. This radius limits the minimum achievable resolution. The minimum line edge roughness (LER) of any feature is defined to the first order by the random generation path of secondary electrons scattered through the resist and the randomized photon distribution. The LER can be visualized as an envelope of a series of circles centered on a single line and just touching the edge of the circle (having a radius equal to the random generation path of scattered secondary electrons). The center of the circle is also statistically defined and determined by the atomic density in the resist. This is shown in FIG.

Secondary electrons not only adversely affect the resolution of the exposure, but can also result in damage to the layers of the integrated circuit underlying the resist layer. There are two possible scenarios for this.
-The voltage of the secondary electrons collected in the resist layer creates a voltage causing damage.
-Secondary electrons enter the selective layer and damage the bonds or structures in that layer.

  The collection of electrons in the resist layer can also adversely affect small structures or structures near large exposed areas. The desired critical dimensions (i.e., the minimum space between two patterns of features such as lines or contacts allowed when creating a device layer and / or the minimum width of a line or any other form) may also vary.

  It is an object of the present invention to at least partially alleviate the above problems.

  According to a first aspect of the present invention, in a method of making a device using a lithographic process, applying a radiation-sensitive resist to the top of a substrate and applying a field to the resist, a portion of the resist Exposing the radiation to radiation, wherein the direction of the electric field is substantially perpendicular to the plane of the resist layer.

  Fabrication methods embodying the present invention tend to prevent secondary electrons from transferring from the exposed areas of the resist to the unexposed areas, thus improving the resolution of the process. Similarly, electrons that leave the resist and migrate to the lower layer of the device can be prevented or reduced.

  In a first embodiment of the invention, the method includes applying a layer of conductive material to the top surface of the resist. During exposure, an electric field is applied, for example by connecting a layer of conductive material to a fixed potential. The conductive material is a metal and may be at least partially transparent to the exposure radiation. One possible conductive material is indium tin oxide.

  The thickness of the conductive layer must be sufficiently thin to minimize the attenuation of transmitted light, for example, less than 10%, and preferably about 2% or less. The layer can be applied to a thickness of less than 50 nm, for example about 10 nm.

  In an alternative embodiment, the method includes providing a layer of conductive material on the underside of the resist between the resist and the surface of the device. The electric field is applied during exposure, for example by connecting a layer of conductive material to a fixed potential. The conductive layer is metallic and may be, for example, indium tin oxide.

  In yet another embodiment, the method includes applying a layer of conductive material to the top surface of the resist, providing a layer of conductive material on the bottom surface of the resist between the resist and the surface of the device, and during exposure. Applying the electric field by applying a potential difference between the two conductive layers.

  In yet another embodiment, the electric field is applied by directly coupling the resist to a fixed potential. For example, a probe connected to a fixed potential is brought into contact with the resist immediately after the start of exposure. It is preferred to incorporate a conductive material into the resist.

  According to a second aspect of the present invention, an illumination system for providing a projection beam of radiation, a support composition for supporting a patterning device to impart a pattern to the projection beam, a substrate table for holding a substrate, and a patterned beam There is provided a lithographic apparatus comprising: a projection system for projecting an image onto a target portion of a substrate; and an electric field generator for applying an electric field across a resist layer provided on a surface of the substrate, wherein the direction of the electric field is substantially perpendicular to the surface of the resist layer The

  According to a third aspect of the present invention, in a method of making a device using a lithographic process, a radiation-sensitive resist containing a conductive material is applied to the top of the device, and an electric field is further applied to the resist. However, a method is provided that includes exposing a portion of a resist to radiation.

  For a better understanding of the present invention and how to achieve it, reference will now be made by way of example to the accompanying drawings in which:

  The following description refers to specific embodiments of the invention, but it will be understood that the invention may be practiced otherwise than as described below. The description is not intended to limit the invention.

FIG. 3 schematically depicts a typical lithographic apparatus. This device
An illumination system IL that supplies a projection beam PB of radiation (eg UV or EUV radiation);
A first support structure (e.g. mask table) MT that supports the patterning device (e.g. mask) MA and is coupled to a first positioning device that accurately positions the patterning device relative to the item PL;
A substrate table (eg a wafer table) WT that supports a substrate (eg a resist coated silicon wafer) W and is connected to a second positioning device PW that accurately positions the substrate with respect to the item PL;
A projection system (eg a reflective projection lens) PL for imaging a pattern imparted to the projection beam PB by the patterning device MA onto a target portion C (eg consisting of one or more dies) of the substrate W;

  As shown here, the apparatus is of a reflective type (eg using a reflective mask or a programmable mirror array of the type as mentioned above). Alternatively, the device may be of a transmissive type (eg using a transmissive mask).

  The illumination system IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is a plasma discharge source. In such a case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is emitted from the source SO to the illuminator with the aid of a radiation collector, such as with a suitable collector mirror and / or spectral purity filter. Passed to IL. In other cases, for example when the source is a mercury lamp, the source may be an integral part of the device. Source SO and illuminator IL may be referred to as a radiation system.

  The projection beam PB is incident on the mask MA, which is held on the mask table MT. The projection beam PB is reflected by the mask MA and passes through a lens PL that focuses the beam onto a target portion C of the substrate W. With the help of the second positioning device PW and the position sensor IF2 (for example an interferometer device), the substrate table WT can be moved precisely, for example to align with different target portions C in the path of the beam PB. Similarly, the first positioning device PM and the position sensor IF1 are used to accurately position the mask MA with respect to the path of the beam PB, for example after mechanical retrieval from a mask library or during a scanning movement. Can do. In general, the movement of the object tables MT and WT is performed by a long stroke module (coarse positioning) and a short stroke module (fine movement positioning) that form parts of the positioning devices PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT is only connected to a short stroke actuator or is fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

  A wafer 1 prepared in advance is shown in FIG. 4a. A thin uniform coating 2 of photosensitive resist is provided on the upper surface of the wafer. The resist layer may have a thickness on the order of 100 nm. In general, the resist layer 2 is generated by rotating a wafer at a high speed and dropping the resist on the wafer surface once or a plurality of times. Next, the rotated resist is baked at a high speed. A thin coating 3 of conductive material (for example to realize a metal layer or a layer containing metal) is provided on the surface of the resist layer 2. The conductive layer is preferably transparent to light at the exposure wavelength. One such material is indium tin oxide. However, this is not essential if the conductive layer 3 is thin enough to allow sufficient light to pass through the resist layer 2. A typical thickness is 10 nm, which results in a light transmission loss of only 2%. The conductive layer 3 can be generated, for example, by vapor deposition or sputtering. If the coating 3 is a metallic layer, an additional advantage is that such layers typically reflect deep ultraviolet (“DUV”) radiation at wavelengths between 130 nm and 400 nm. In general, EUV radiation sources also emit such DUV radiation. Resists, especially chemically amplified resists, are very sensitive to DUV radiation. As a result, the imaged pattern may have flare or ghost images during exposure, which degrades the resolution performance of the EUV lithographic apparatus. The metallic topcoat reduces or substantially suppresses the presence of the DUV radiation in the resist layer during exposure, thereby mitigating flare and / or ghost image problems.

  FIG. 4 b shows an alternative preconditioned wafer configuration, in which a second layer of conductive material 4 is provided between the resist layer 2 and the surface of the wafer 1. Since this layer does not need to transmit light, the material used and the properties of the layer itself may differ from those of the conductive layer 3 above.

  FIG. 4 c shows another alternative preconditioned wafer configuration that incorporates a conductive material into the resist material that provides the resist layer 2. This conductive material may be, for example, silicon. A further coating 5 is provided on the surface of the resist. This coating is preferably formed of AIN (aluminum nitride), Ru, Ir, Au, SiN, Rh, Si or C and provides outgassing protection for the lithographic apparatus. This will be further described below. A barrier layer 6 is additionally provided between the wafer 1 and the resist layer 2 on the surface of the wafer 1. This layer 6 helps to prevent conductive ions from migrating from the resist layer into the wafer, and without it, migration problems can occur and can cause damage to the wafer.

  After adjusting the pre-adjusted wafer, the wafer is introduced into the apparatus of FIG. 3, and the substrate table WT is positioned in a state aligned with the mask MA at an appropriate position with respect to the projection system PL. In the case of the wafer shown in FIG. 4 a, the probe is brought into conductive contact with the conductive layer 3. The probe connects to ground potential and thus connects the conductive layer 3 to ground. The wafer is then illuminated with a suitable light pattern. It is understood that free electrons such as secondary electrons generated during the exposure step tend to move in the direction toward the ground layer 3. That is, the electrons tend not to move in the horizontal plane (relative to FIG. 4a), thus reducing the exposure of non-illuminated areas.

  Referring to the wafer of FIG. 4b, after aligning the wafer with projection system PL, it is in conductive contact with both conductive layers 3 and 4. A static (DC) potential is applied between the two layers so that the upper layer 3 is at a positive potential relative to the lower layer 4. Again, this has the effect of moving the free electrons towards the upper layer 3 and preventing lateral movement. In an alternative configuration, the polarity of the potential is opposite to that of FIG. This has the effect of accelerating free electrons toward the wafer surface and effectively improving the sensitivity of the resist.

  Referring to the wafer of FIG. 4c, it is in conductive contact with the resist layer 2 so that the layer can be bonded to ground.

  FIG. 6 is a flowchart showing the important steps of the device creation procedure.

  It will be appreciated that coupling the probe to the conductive portion of the device (which may be a resist or an additional conductive layer) proves difficult in practice. A better solution is to allow the conductive material to overlap the side of the wafer and extend around the base of the wafer. An appropriate electric field can be applied to the resist layer 2 by coupling the base to a fixed potential.

  The problem of degradation of the optical components of the lithographic apparatus when operating with EUV radiation is discussed, for example, in US Pat. No. 6,459,472. This appears to be caused by the release of material gas from the surface of the resist layer that forms the coating on the final optical component (mirror or lens). Of particular concern are hydrocarbons and sulfur. Also, outgassing of resists containing silicon is undesirable (eg, to improve resist transparency at the exposure wavelength). Resists containing silicon tend to release silicon during exposure and optical elements are degraded by free Si or Si ions. This problem is expected to be greatly reduced if the resist layer 2 is coated with a top layer that is dense enough to prevent outgassing from the resist. For example, the arrangement of FIGS. 4a and 4b (or FIG. 5) is particularly suitable. If a conductive top layer is not provided as part of the means for reducing the effects of secondary electrons (eg, FIG. 4c incorporating a conductive material into the resist), it is preferable to add an additional outgassing protective layer 5. Needless to say, it must be thin enough to avoid significant absorption of light in the outgassing protective layer.

  U.S. Pat. No. 6,459,472 presents a solution to the problem of removing sputter debris from the surface of a wafer, which includes a channel containing argon gas for the final optical components of the lithographic apparatus and the wafer. Must be introduced between the surface. Argon has a much lower EUV absorption than air. The chamber gas is continuously flowed to remove sputtering debris from the wafer surface. A potential problem with this method is that argon atoms are ionized to some extent by illumination radiation. When the top conductive layer of the resist is coupled to ground, the argon containing chamber can be connected to a positive potential. This is shown in FIG. With this arrangement, ions are released from the resist material (which tends to be positively charged) and are attracted towards the wafer away from the projection optics, thus reducing the level of contamination that the optics suffer. .

  Those skilled in the art will appreciate that various modifications can be made to the above-described embodiments without departing from the scope of the present invention.

2 shows a secondary electron generation mechanism in a photoresist. FIG. 2 is a diagram showing a limit to line edge roughness in a photolithography process. 1 shows a diagram of a typical lithographic apparatus. a through c illustrate various procedures for improving the resolution of the apparatus of FIG. Fig. 4 shows an alternative procedure for improving the resolution of the device of Fig. 3; 2 is a flow diagram showing process steps selected in the creation of an integrated circuit. Fig. 4 shows a further alternative procedure for improving the resolution of the device of Fig. 3;

Claims (13)

A method of manufacturing a device using a lithographic process comprising:
Exposing a portion of the resist layer on the substrate to radiation;
Applying an electric field to the resist layer , and during exposure, the direction of the electric field is substantially perpendicular to the plane of the resist layer ;
further,
Applying a layer of conductive material to the top surface of the resist;
Providing a layer of conductive material between the lower surface of the resist and the surface of the substrate;
Applying the electric field during exposure by applying a potential difference between two conductive layers . The method of claim 1 , wherein the conductive material is metallic. The method of claim 1 , further comprising applying the layer of conductive material at a thickness of less than 50 nm. The method of claim 1 , wherein the layer of conductive material overlaps a side of the substrate and extends into at least a portion of the base of the substrate.   The method of claim 1, wherein the radiation is in the extreme ultraviolet range.   The method of claim 1, further comprising orienting the electric field such that the upper surface of the resist layer is at a positive potential relative to the lower surface of the resist layer. The method of claim 1, further comprising orienting the electric field such that the upper surface of the resist layer is at a negative potential relative to the lower surface of the resist layer. A method of manufacturing a device using a lithographic process comprising:
Exposing a portion of the resist layer on the substrate to radiation;
By binding directly to the conductive material incorporating a layer of resist to a fixed potential, and a applying an electric field to the resist layer during exposure, the direction of the electric field, Ri substantially perpendicular der to the plane of the resist layer,
And a barrier layer for preventing migration of conductive ions from the resist layer toward the surface of the substrate. The method further includes providing a barrier layer between the lower surface of the resist layer and the surface of the substrate. 9. The method of claim 8 , wherein a resist layer incorporating a conductive material overlaps a side of the substrate and extends into at least a portion of the base of the substrate. 9. The method of claim 8 , further comprising applying a layer that reduces outgassing to the top surface of the resist. 11. The method of claim 10 , wherein the layer that reduces outgassing comprises one of aluminum nitride, Ru, Ir, Au, SiN, Rh, Si, or C. Lithographic equipment,
An illumination system for supplying a projection beam of radiation;
A support structure that supports a patterning device that imparts a pattern to the projection beam;
A substrate table for holding the substrate;
A projection system for projecting the patterned beam onto a target portion of the substrate;
Is configured to apply an electric field to the resist layer provided on the surface of the substrate, and a deployed field generator, the direction of the electric field, Ri substantially perpendicular der to the plane of the resist layer,
The electric field generator applies an electric field to the resist layer by applying a potential difference between the conductive layer provided on the upper surface of the resist layer and the conductive layer provided between the lower surface of the resist layer and the surface of the substrate. Lithographic apparatus. Lithographic equipment,
An illumination system for supplying a projection beam of radiation;
A support structure that supports a patterning device that imparts a pattern to the projection beam;
A substrate table for holding the substrate;
A projection system for projecting the patterned beam onto a target portion of the substrate;
Is configured to apply an electric field to the resist layer provided on the surface of the substrate, and a deployed field generator, the direction of the electric field, Ri substantially perpendicular der to the plane of the resist layer,
The electric field generator applies an electric field to the resist layer by directly coupling a resist layer incorporating a conductive material to a fixed potential;
Furthermore, a lithographic apparatus, wherein a barrier layer is provided between the lower surface of the resist layer and the surface of the substrate to prevent movement of conductive ions from the resist layer toward the surface of the substrate .

Images